The thing that is most obvious when examining the two cards of the title is that the figure on each is manipulating a fluid that has been symbolically linked to the ethereal elixir of Spirit. Temperance is sloshing it back-and-forth between two chalices (and much has been made of the fact that, in the Waite-Smith image, the remarkable liquid is falling – and rising – diagonally and not vertically, apparently immune to the effects of gravity, thus testifying to its rarefied nature), while the Star shows it being poured from two vases onto water and land, anointing both equally with its spiritual largesse.
It seems to me that Temperance is concentrating, conserving and purifying that elixir. In old metal-working terms, to “temper” a metal meant to refine it into a more durable state through a physical transformation at the molecular level, and the alchemical process of transmutation that esoteric writers ascribe to Temperance means much the same thing. Temperance isn’t a card of “moderation,” telling us to pull in our horns, keep our feet under us, and take a more balanced, restrained stance, it is one of “modulation,” which means to “adjust,” “attune” or “harmonize.” It has to do with bringing subtle forces into alignment through an act of Will, which is why I consider it a card of “finesse,” signifying the “Fine Art of Right Action.” It is cautionary in advising a kind of tightrope walk between applying too much force and too little, but not in the sense that we should step back from confronting the challenge.
The Star, on the other hand, is a card of introspection and inspiration; to inspire (or “in-spirit”) a thought is to endow it with noble vision that is generally seen as deriving from a higher source. It suggests the “Aha!” moment when a fuzzy outlook snaps into focus with great clarity and precision. When that epiphany can be brought down to Earth, it offers the realization of one’s fondest hopes; when it can’t, it amounts to little more than wishful thinking of the Joe Hill “pie-in-the-sky” variety. The flowing water represents the outpouring of illumination from above; unlike in the Magician, it’s not a consciously channeled and directed current but more of a spontaneous gusher. At its best it stimulates aspiration to “reach for the sky,” at its worst, it leads down a rabbit-hole of misplaced optimism (especially if it turns up reversed or otherwise ill-dignified in a reading).
The main difference I see between the expression of Spirit in these cards is that Temperance presents an active scenario that demands engagement with its principles, whereas the Star is much more passive, imparting its infusion of visionary insight in a benign, meditative way that could even be seen as a mystical extension of the water flowing away from the feet of the High Priestess, the exalted agency for all such things.